The use of natural gas as an engine fuel source has been recognized to have many advantages. Natural gas is a clean burning fuel which promotes engine cleanliness and lowers overall tailpipe emissions. It may also be used as a fuel without the addition of the additives required in gasoline, for example, which often include heavy metals. To date, most commercially viable technology used for converting Otto-cycle engines to operate on gaseous fuels are mechanically controlled systems based wholly or partly on apparatus and methods conceived and designed in Italy in the 1920's. Such mechanical systems are incapable of meeting modern vehicular engine requirements. In particular, they fail to provide the responsiveness, power, or fuel efficiency expected by drivers or the exhaust emission levels now legislated by many regulatory authorities.
To overcome this deficit, modern and sophisticated electronically controlled systems have been invented for converting liquid fuelled engines to gaseous fuels. The most notable of those inventions are briefly discussed below.
U.S. Pat. No. 5,092,305 which issued on Mar. 3, 1992 to King is entitled "APPARATUS AND METHOD FOR PROVIDING AN ALTERNATIVE FUEL SYSTEM FOR ENGINES". This patent describes an alternate fuel system that operates in conjunction with the primary fuel system to utilize the output from the existing original equipment manufacturer's control module in the primary system. It modifies the original equipment manufacturer's control signals to operate a fuel supply valve for the alternative fuel so that the proper quantity of alternative fuel is supplied to the engine. Spark control is supplied by the original equipment control module. There are several drawbacks to this approach. One of the principle drawbacks is that a gaseous fuel such as natural gas performs differently than liquid fuels such as gasoline. Because of this, different fuelling is required during various engine operating modes such as cold start, warm-up, power enrichment and transient periods during which different torque requirements may be necessary. In addition, gaseous fuels require different ignition timing control than liquid fuels. Since this system relies on the original equipment manufacturer's timing signals for ignition control, optimal performance and minimal emissions cannot be realized. Engine spark timing must be advanced significantly when running on natural gas to compensate for the fact that natural gas burns more slowly than gasoline and therefore needs to be ignited earlier in order to exert maximum mean pressure downward on the piston.
U.S. Pat. No. 5,353,765 which issued Oct. 11, 1994 to Saikalis et al is entitled "FUEL MANAGEMENT SYSTEM FOR A GASEOUS FUEL INTERNAL COMBUSTION ENGINE". This patent teaches closed loop fuel control using a gas flow fuel sensor, a mass flow air sensor and exhaust O.sub.2 sensor to control a PWM injector signal for injecting gaseous fuel into the intake manifold of a converted engine. The mass gas flow sensor is fluidly connected between the gaseous fuel source and the engine. The mass airflow sensor is similarly provided in series with an air intake for the internal combustion engine and the outputs of both flow sensors are provided as input signals to a microprocessor based control circuit of the invention. The control circuit calculates the air/fuel ratio of the combustible charge provided to the engine and generates appropriate output signals to a valve arrangement in series between the source of gaseous fuel and the internal combustion engine to vary the air/fuel ratio to obtain a desired engine performance as a function of the engine operating condition. It is unclear whether this system would provide suitable dynamic response in fuel requirements for widely and rapidly varying engine power demands. Furthermore, requirements for the control, monitoring and safe operation of the gas flow sensor appear to be complex in both software algorithms and hardware requirements. This system also fails to address the issue of optimum spark timing while in a gaseous fuel mode.
U.S. Pat. No. 5,367,999 which issued Nov. 29, 1994 to King is entitled "METHOD AND SYSTEM FOR IMPROVED FUEL SYSTEM PERFORMANCE OF A GASEOUS FUEL ENGINE". This patent relates to an improved gaseous fuel delivery system and method of improving performance of the gaseous fuelled engine by variably maintaining gaseous fuel pressure to the engine. An electronically controlled pressure fuel regulator receives signals from a fuel composition sensor, a fuel temperature sensor and a fuel pressure sensor as well as an engine speed or manifold absolute pressure signal to control an electronic fuel pressure regulator in order to improve the fuel system dynamic range to provide improved fuel metering accuracy and improved engine performance and emissions. While this appears to be a novel concept for improving fuel metering, it is believed that fuel metering is more accurately and economically controlled by precise control of a high speed electronic injector. This patent also fails to address ignition control, as is common in the prior art.
U.S. Pat. No. 5,598,825 which issued Feb. 4, 1997 to Neumann is entitled "ENGINE CONTROL UNIT". This patent relates to an electronic control unit used for converting two cycle turbo-charged diesel engines to operate on natural gas. The electronic control unit detects engine speed, throttle position, manifold absolute pressure, gas pressure, gas temperature, battery voltage, air temperature, engine phase and boost pressure control valve feedback position to control the gaseous injector ON time. The control unit calculates a percentage full-load value for the engine based on detected engine speed and throttle position and utilizes a calculated percentage full-load value to calculate the injector ON time for each gas injector in a gas delivery system for the engine. Typically the control unit employs the percentage full-load value and the engine speed to calculate a required manifold absolute pressure value, and this calculated manifold absolute pressure value may then be employed together with the percentage full load value to calculate a percent allowable load value. The engine control unit employs the percentage allowable load value with current engine speed to calculate the spark advance for a given intake air temperature by reference to a look-up table stored in the ECU controller. Empirically mapped data in the 3-dimensional table provides limited range control for optimal spark advance. While spark advance is advantageous for optimal engine performance, complete ignition control, including ignition coil dwell time control is required for optimal engine performance and minimum tailpipe emissions.
In view of the above discussed patents, it is clear that while significant advances have been made in systems for converting liquid-fuelled engines to operate on gaseous fuels, an economical conversion system which provides the engine performance expected in modern vehicular applications has not been realized in the prior art.